Extruders are commonly used in order to form various types of products (e.g., blown film, pipe, coated paper, plastic filaments, carpet fiber, and vinyl siding) from thermoplastic materials, such as polyvinylchloride, polyethylene or polypropylene, for example. The extruder carries out an extrusion process whereby a solid plastic (or resin), which is often in the form of beads, pellet or powder form, is compressed, melted (plasticized) and mixed while being carried along by a feedscrew, after being fed into a barrel chamber which houses the feedscrew. Ultimately, a resin, which is expectantly homogenized, is forced out of the extruder through a die, which, depending on its design and other downstream equipment, forms the resin into the particular desired shape.
Depending on the application, the plastic material is often compounded with other chemicals prior to formation of the plastic material into pellets or beads. These chemicals, which may vary in amount, include, for example, stabilizers, lubricants, dyes or other colorants, plasticizers, fillers, and other additives which may add or enhance certain desirable product properties. In order to obtain the desired properties throughout the final product, it is important that the additives, such as those listed, are evenly distributed and dispersed in the extrudate as it enters the die.
Referring to FIG. 1, a prior art single screw extruder machine 5 is shown comprising a barrel 12, a feedscrew 14, an adapter 16, a die 18, and a hopper 19. The barrel 12 is typically a thick-walled steel chamber that is machined to have a tight fit with the feedscrew 14. The feedscrew 14, which is usually the only moving part in the extruder, generally includes at least three defined sections: a feed zone, a compression zone, and a metering zone, each zone varying, for example, in channel depth, distance between flights, and flight patterns. The feed zone takes resin from a hopper 19 and conveys it along the initial part of the barrel chamber. During this conveyance, the resin pellets encounter friction from the feedscrew surface, the barrel surface and from other pellets, resulting in heat which begins to melt the resin. In the compression zone, the channel depth between screw flights usually diminishes resulting in the pressurization of the melting resin and hopefully the completion of the melting process. The metering zone generally serves to further mix the molten resin, hopefully resulting in a homogenized resin with a uniform temperature.
The feedscrew 14 typically has a smooth conical tip, which is enclosed within the adapter 16. The adapter 16 serves as a connection between the barrel 12 and the die 18. It often houses a breaker plate (which breaks the helical flow of the molten resin into a linear flow) and a screen pack (which filters out foreign particles).
Generally, there are two important types of mixing that the screws effectuate in an extruder in order to reduce the non-uniformity of the material or materials being extruded. These two types of mixing, which basically serve to induce relative physical motion in the ingredients of the extrudate, are distributive mixing and dispersive mixing. The combination of both dispersive and distributive mixing will achieve a more uniform overall mixture.
Distributive mixing increases the randomness of the spatial distribution of particles without reducing the size of the particles. It is effectuated by a continuous reorientation of the material being mixed.
Dispersive mixing serves to reduce the size of cohesive particles as well as randomizing their positions. In extrusion processes, dispersive mixing is usually more important than distributive mixing. This is especially true, for example, in the extrusion of compounds which contain colorant pigment agglomerates, which must be uniformly mixed into the extrudate. In dispersive mixing, solid components, such as agglomerates, or high viscosity droplets, are exposed to sufficiently high stresses to cause them to exceed their yield stress, thus causing the agglomerates or droplets to be broken down into smaller pieces. Depending upon the type of material being extruded, including the size, shape, and chemical bonding, varying amounts of stress will be required to break up these materials. The stress which breaks up the agglomerate may either be shear stress or elongational stress. Generally, elongational stress, which is generated by elongational flow or stretching, is more efficient in effectuating dispersion of the material than shear stress, which is caused by the sliding of the material across itself.
In order to enhance the distributive and dispersive mixing in extruders, extruder screws having numerous variations in design have been employed. Extruder screws usually have a standard screw section near the material input hopper, and one or more specially designed sections to enhance distributive and dispersive mixing. To enhance distributive mixing, for example, distributive mixing elements such as pins placed between screw flights, or channels which divide the polymer flow into many narrow channels which are combined and re-divided, are employed. These types of elements generally do not provide regions of high stress and thus serve mainly to spatially redistribute material.
In order to enhance dispersive mixing, i.e., to break up and disperse the agglomerates, screws are employed which have high shear stress or elongational stress regions. For example, screws may be used having a fluted or splined mixing section, in which one or more barrier flights or blister rings are placed along the screw so that the material has to flow over the barriers.
The above described distributive and dispersive mixing elements or regions are typically located at one or more areas on the screw, and thus the mixing is performed in the barrel of the screw extruder. Little, if any, mixing typically occurs in the area of the adapter, which typically encloses the screw tip. As such, the adapter is a location where stagnation of the molten resin may occur potentially resulting in incomplete mixing, heat gradients, and therefore decomposition, degradation or burning of the material, as well as a generation of excessive head pressure, which limits output.
U.S. Pat. No. 3,942,774 to Sokolow describes a screw tip located in an adapter of a screw extruder which includes transfer grooves for enhancing redistributive mixing in the adapter area. The transfer grooves of the screw tip work in combination with feeder grooves in the inside surface of the adapter to effectuate the redistributive mixing. The constraint of having to replace the adapter in order for the screw tip to work effectively inhibits the ability of the screw tip to be used with existing adapters.
What is desired is an improved screw tip for extruders.